Widely spread image forming apparatuses such as a copying apparatus, printer, and facsimile apparatus have a paper storage portion (paper feed stage or paper feed portion) such as a cassette or tray which stores print sheets. Such image forming apparatus forms an image on a paper sheet supplied from the paper storage portion, and discharges the paper sheet outside the image forming apparatus. In order to form images on various paper sheets, some image forming apparatuses have a plurality of paper storage portions and can store paper sheets of various sizes and various types in the respective paper storage portions. At present, image forming apparatuses which store many paper sheets of a specific size by storing paper sheets of the same size in a plurality of paper feed stages, and form many images by one paper replenishment have become popular.
An image forming apparatus which stores paper sheets of the same size in a plurality of paper feed stages has an auto cassette change (hereinafter referred to as ACC) function of, when all paper sheets in a paper storage portion are consumed but paper sheets of the same paper size are stored in another paper storage portion, keeping feeding paper sheets from the substitute paper storage portion without stopping paper feed operation. Even if paper sheets in a given cassette run out, paper sheets in another cassette can be successively used. This shortens the operation stop time due to the absence of paper sheets, and increases the availability and productivity of the image forming apparatus (see Japanese Patent Laid-Open No. 2002-40881).
Japanese Patent Laid-Open No. 2002-40881 also discloses an image forming apparatus which considers the paper type in addition to the paper size. The image forming apparatus determines whether a paper storage portion storing paper sheets of the same size and same paper type as paper sheets which run out is available and, when no such paper storage portion, the auto cassette change for paper sheets which run out is not performed.
There is also proposed an image forming apparatus which stores paper sheets called index sheets (tabbed sheets) in the paper storage portion of the image forming apparatus, which may form images (see Japanese Patent Laid-Open No. 8-73110). As disclosed in Japanese Patent Laid-Open No. 8-73110, an index sheet (tabbed sheet) has an index (tab) added to a normal rectangular paper sheet. In general, index positions sequentially shift, the sheets overlap each other, and a plurality of index sheets form one set. The number of index sheets which form one set will be called a division number in this specification. Some image forming apparatuses which feed and deliver index sheets have a function of automatically discharging excessive index sheets out of a bundle of index sheets of one set from the paper storage portion outside the apparatus (see Japanese Patent Laid-Open No. 2002-3063).
A printed index sheet is set on a post-processing device called an inserter or collator in a copying apparatus or printer. The index sheet is inserted at a designated position of a printed or copied document, or manually inserted into a document. While a document is printed or copied on normal paper sheets, an index sheet may be conveyed to a designated insertion position from a paper storage portion which supports the index sheet, subjected to printing, and then merged into the printed or copied document.
When a paper storage portion whose paper size and paper type coincide with those of a paper storage portion which runs out of paper sheets is designated as the change destination of the ACC function, as disclosed in Japanese Patent Laid-Open No. 2002-40881, an image forming apparatus which feeds index sheets, as disclosed in Japanese Patent Laid-Open No. 2002-3063, performs auto cassette change even with a different index number (division number) of index sheets. Particularly, an index sheet is larger in width by the index than a normal paper sheet. Index sheets may not be stored in a normal paper feed stage, and can only be stored in a limited paper feed stage.
For example, in an image forming apparatus shown in FIG. 2, cassettes 311 and 312 are paper feed stages which can store many paper sheets but cannot store index sheets. Cassettes 313 and 314 are paper feed stages which can store a smaller number of paper sheets, compared to the cassette 311, but can store even index sheets. In this case, a user who uses many index sheets can store index sheets in both the cassettes 313 and 314, perform auto cassette change between index sheets, and use the index sheets. When, for example, the cassette 313 stores index sheets with an index number (division number) of 4 and the cassette 314 stores index sheets with an index number (division number) of 5, these index sheets are of originally different paper types, and auto cassette change between these cassettes is not intended by the user. In this case, it is preferable not to perform auto cassette change.
When index sheets with an index number of 4 are stored in both the cassettes 313 and 314, auto cassette change is preferably performed because of the same index number.